Question

A 2.20 g sample of a compound gave 5.63 g CO2 and 2.30 g H2O on combustion in air. The compound is known to contain only C, H, O. What is its simplest formula?

Answer 1

The simplest formula is C₅H₁₀O.

We must calculate the masses of C, H, and O from the masses given.

Mass of C =5.63 g CO₂ × (12.01 g C/44.01 g CO₂) = 1.536 g C

Mass of H = 2.30 g H₂O × (2.016 g H/18.02 g H₂O) = 0.2573 g H

Mass of O = Mass of compound - Mass of C - Mass of H

= (2.20 – 1.536 – 0.2573) g = 0.406 g

Now, we must convert these masses to moles and find their ratios.

From here on, I like to summarize the calculations in a table.

Element Mass/g Moles Ratio Integers

C 1.536 0.1279 5.038 5

H 0.2573 0.2553 10.05 10

O 0.406 0.0254 1 1

The empirical formula is C₅H₁₀O.